1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to signal filtering and voltage limiter circuits for use in a wideband wireless communication systems.
2. Description of the Related Art
Super-heterodyne receivers traditionally receive a Radio Frequency (RF) signal that must be converted to baseband by way of an intermediate frequency (IF). Thereafter, the IF signal is amplified. In a transmitter, similarly, a baseband signal often is up converted to the intermediate frequency wherein the amplification and subsequent filtering are carried out at the IF stages. While some narrow band systems skip the IF conversion step, wideband systems typically require conversion to IF stages. Depending on the signal bandwidth and the type of communication system, most semiconductor devices are not yet able to allow full integration of active filters operating at the elevated intermediate frequencies for a wideband or high data rate communication network.
Some narrow band or low data rate systems, such as Bluetooth, use a low intermediate frequency design approach. This approach is advantageous in that it facilitates the design of the IF portion of a radio on the integrated circuit device thereby allowing the development of low power circuitry that can be placed in new applications not seen before. Many transceiver devices utilize principles of frequency discrimination in order to facilitate the frequency spectrum being used by a plurality of users. Known frequency discrimination techniques include older systems that dedicate at least one frequency for a communication link between two wireless transceivers while other systems use a combination of time and frequency discrimination. One example of such system is the North American Time Division Multiple Access (TDMA) scheme in which communication slots are characterized by frequency and time in relation to a synchronization signal. Other known radio systems include Global System for Mobile Communications (GSM) wireless communication systems that also are TDMA-based systems.
These communication systems, as they become more popular, tend to experience greater levels of interference from other users, as well as from environmental conditions. For example, multi-path interference results in part from the reflection of signals off of physical structures, which reflections interfere with the primary signal. Additionally, electronic noise sources also create interference. Because of the noisy environments that therefore exist in the wireless communication mediums, radios are built to include multiple processing steps to extract and purify a signal.
For example, a Bluetooth radio transmits a communication signal with a 2.4 GHz center frequency and with a 1 MHz band. Because radios actually process the data at baseband, however, such Bluetooth systems typically down convert from the transmission frequency of 2.4 GHz to an intermediate frequency prior to converting the signals down to baseband. Additionally, while the signals are at the intermediate frequency stage, significant processing occurs to eliminate noise and interference prior to converting the signals to baseband. Thus, because of frequency drift and other known problems, the signals are mixed with a local oscillator prior to conversion to baseband. Careful signal processing at this intermediate frequency stage allows for the greatest signal-to-noise ratio and therefore the purist signal for processing at the baseband level once the final conversion step occurs.
As a part of reconstructing the signal at the intermediate frequency stage, an amplifier is used for significantly amplifying very low voltage signals that are received so that they may be processed. To accurately determine the signal frequency and to reconstruct it, however, the rising and falling edges of the signal should be determined as precisely as possible.
Generally, signals are amplified for processing using one of several different amplifiers. Limiting amplifiers are sometimes used because they are operable to amplify a signal to reach and (perhaps exceed) a specified voltage or gain level. One problem with the majority of known limiting amplifiers, however, is that they have a coarse level of control of the final output amplitude. Most limiting amplifiers merely amplify a detected and received signal to a maximum value and then allow the amplified signal to clip at a specified value. While this design approach is acceptable for many systems, such a design approach has the adverse affect of causing the output stages of the amplifiers to cut off while a signal is being clipped. Accordingly, once the clipping terminates, a response period is required for the output stages of the amplifier to become present and operational again. Thus, an overload voltage range can result in quantization failures. There is a need, therefore, for a limiting amplifier in a wireless transceiver system that provides a precise output limit in the final amplification stages that maximizes the amplification of the signal while avoiding clipping and the adverse effects therefrom.
The present invention provides a circuit formed within a low power CMOS integrated circuit and a method for limiting a voltage to a specified value (e.g., a rail voltage) without clipping, thereby avoiding the undesirable consequences that result from clipping such as turn off of the output stage amplifiers. In general, a circuit is provided that adds current or removes current from an output resistive load so that the output voltage developed across the load will remain within defined limits. More specifically, in one embodiment of the invention, a Boll and Cascode configuration is used to steer the current in and out of the resistive load. A pair of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) is biased to turn on when a specified output voltage is reached to either add to or sink current from the input node of the resistive load thereby maintaining the output voltage in a predefined range. A plurality of biasing circuits is provided that control the turn on voltage levels for the MOSFETs to achieve the desired operation.
In general, the invention includes a differential MOSFET pair configuration that provide current amplification that are connected to steering circuitry for steering current in and out of the resistive load coupled across the differential MOSFET pair. The steering circuitry then is coupled to receive biasing signals from biasing circuitry that sets the upper and lower rail voltage limits. As such, current is removed (steered out of the resistive load) when an amplified signal is tending to exceed an upper rail voltage limit as defined by a first bias signal. Conversely, current is added (steered into the resistive load) when an amplified signal is tending to exceed a lower rail voltage limit as defined by a second bias signal. The biasing circuits include circuit components that are matched to circuit components within the voltage limiting circuitry to add and remove current to the output resistive load. The biasing circuits each further include a replica resistive load (resistor in one embodiment of the invention) that matches the output resistive load. Accordingly, the accuracy of the output saturation limits of the amplifier can be precisely controlled to better than 1% variation with the described circuitry. This control of the output leads to a more optimal system design as the following stages can be optimized without requiring significantly extra dynamic range. Subsequently the receiver system can be made more robust.
Other aspects of the present invention will become apparent with further reference to the drawings and specification, which follow.